1. Technical Field
The present invention relates to a method of modifying qualities of a metallic article at its surface utilizing high density energy, and an apparatus therefor.
2. Background Art
There is an increasing need for strengthening metallic parts, such as aluminum castings, at their local parts, particularly at specified locations on their surfaces In the automotive parts industry, for example, a cylinder head made of aluminum or cast iron casting should bear greater thermal load as well as mechanical load as engine output increases, because its operating temperature reaches a considerably high temperature. Such an increase in load on the engine parts results in unexpected thermal cracking, as shown in FIG. 12, between an intake port 3 and an exhaust port 4 (referred herein as "intervalve part 6", which is thinner than other portions), and between the intervalve part 6 and a fuel injection nozzle port 5 (or the preheater chamber port). In addition, as shown in FIG. 13, a hatched portion 9 of a cylinder 7, which surrounds a combustion chamber 8 is also an object part since the hatched portions 9 bear a consideraly high load.
As for the methods of modifying qualities of the object parts, there is known a method called the "remelting method". This method is reported to attain a certain degree of satisfaction in improving the resistance to thermal cracking [See Kanazawa, Miyake and two others; "Development of aluminum alloy cylinder head for improving strength of its intervalve part by TTG remelting method", Toyota Gijutsu, Vol. 37, No. 2 (1987), pp. 112-119].
Many attempts have been made to supersede this method. The resultant techniques, which may be referred to categorically as the "remelting alloying method", are reviewed below in terms of their technical contexts and deficiencies:
1. The TIG welding method: While performing the remelting with a TIG welder, suitable heat resisting metal or alloy is supplied as the alloying additive in powdery form to the TIG arc through a powder feeder. Since the amount of the alloying additive that can be added to is 10% at the most by the volumetric ratio, however, not much improvement can be achieved. If the addition is increased forcibly, moreover, the gas that the matrix (Al casting) liberates on alloying gets entrapped in the weld molten pool, forming and leaving there the gas defects, typically porosity due to formation of blowholes.
2. The electron beam remelting method: The alloying elements are either placed on the object part beforehand or fed thereto in the form of filler wire, and are remelted with an electron beam. The operation must be conducted in vacuo, meaning that the supply of the additive elements is both difficult and costly not only in operational practice but also mechanically; if powdery, moreover, they must be prevented somehow from scattering on the one hand, and on the other hand, the surfaces of the work piece other than the object parts must be protected from getting contaminated with the scattered powder.
3. The laser method: Although lasers can be used in the ambient air, both the object part and the additive elements must either be sufficiently dark in color or be coated over with a black paint, lest the absorption of the laser ray should be insufficient. Besides, laser-based apparatuses are still too expensive today.
4. The plasma welding method: This method operates in much the same way as the TIG method, except that the depth of penetration is some 20-30% less here. The TIG's fault of overzealous addition resulting in formation of the gas defects is left unresolved.
As reviewed above, the currently available methods cannot are not practical because of their characteristic faults.